It is an object of the present invention to provide a bidirectional multichannel telecommunication system, a bidirectional optical amplifier, and a method for the bidirectional transmission of optical signals.
In the latest telecommunication technology, it is known to use optical fibers to send optical signals carrying information for long-distance communication.
It is also known that optical signals sent in an optical fiber undergo attenuation along the way, making it necessary to amplify the signal so that it will travel the entire required distance and reach the receiving station at a power level sufficient for correct signal reception.
Said amplification may be effected by means of appropriate amplifiers placed at predetermined intervals along the line, which periodically boost the power of the transmitted optical signal.
Optical amplifiers are suitably used for this purpose, by which the signal is amplified while remaining in optical form, i.e. without the optoelectronic detection and electrooptical regeneration of same.
Said optical amplifiers are based on the properties of a fluorescent dopant (e.g. erbium) which, if appropriately excited by the application of luminous energy, provides a strong emission in the wavelength band corresponding to the minimum attenuation of light in silica-based optical fibers.
Said amplifiers are unidirectional devices, i.e. having a predetermined direction of travel of the optical signal inside them.
This is due, as described for example in U.S. Pat. Nos. 5,204,923 and 5,210,808 of the Applicant, to the fact that the optical amplifiers, particularly if high gain values are required, incorporate unidirectional components that prevent signals reflected outside the amplifiers, e.g. due to Rayleigh scattering along the optical line fibers connected to the amplifiers, from returning into the amplifier, causing interferometric noise.
For the bidirectional transmission of optical signals, known technology generally calls for the use of two separate communication lines, equipped with their respective amplifiers, each of which is used to communicate in a single direction. This results in a high connection cost.
Nevertheless, some technologies are know whose objective is to permit bidirectional transmission on fiber-optic lines by means of bidirectional optical amplifiers.
Bidirectional amplification schemes have been presented with the use of a single unidirectional amplifier that exploit the possibility of fluorescent doped amplifiers to independently amplify signals at different wavelengths.
A bidirectional amplifier based on this principal is described in the article by S. Seikai et al.: xe2x80x9cNovel Optical Circuit Suitable for Wavelength Division Bidirectional Optical amplificationxe2x80x9d, published in Electronics Letters, vol. 29, no. 14, Jul. 8, 1993, pages 1268-1270. It discusses a device placed along a fiber-optic transmission line in which two signals with different wavelengths propagate in opposite directions and which consists of wavelength selective couplers and a known type of unidirectional doped-fiber amplification unit connected by sections of passive optical fiber. The wavelengths of the signals are both internal to amplification band of the doped fiber. By means of selective couplers the two signals at different;wavelengths are input to different optical paths. The two optical paths coincide only in the section corresponding to the amplifying fiber, which is passed through by the two signals in the same direction. The device has a problem of instability caused by internal reflections at a wavelength between those of the propagating signals, a problem resolved through the addition of filters, some of them adjustable, which results in a highly complicated structure and the need to use devices to accurately and continuous adjust said filters.
Patent application EP96100586, filed on Jan. 17, 1996 in the name of the Applicant, describes, among other things, a bidirectional optical amplifier comprising:
an optical amplification unit including at least an optical isolator,
two optical input and output ports for at least two optical signals having opposite propagation directions, said signals having, respectively, a first and a second distinct wavelengths,
two first and two second wavelength selective optical couplers, having respectively a first wavelength passband, including said first wavelength, and a second wavelength passband, including said second wavelength, with no overlapping,
said amplification unit being connected between two opposite nodes of an optical bridge circuit, to whose other opposite nodes said input and output ports are connected, and said first and second selective optical couplers being present at the nodes of said bridge circuit, in which said first and second selective couplers are arranged symmetrically with respect to the amplification unit and with respect to the input and output ports of said optical signals.
Bidirectional amplification schemes have also been presented with separation of the counterpropagating signals and the use of a unidirectional amplifier for each direction.
As an example, the article by C. W. Barnard et al. xe2x80x9cBidirectional Fiber Amplifiersxe2x80x9d , published in IEEE Photonics Technology Letters, vol. 4, no. 8, August 1992, pages 911-913, describes bidirectional erbium-doped fiber amplifier repeaters for bidirectional fiber networks and OTDR fault detection. At each repeater the counterpropagating signals are separated, amplified separately, then recombined. Signal separation is done by a bidirectional fiber coupler or an optical circulator. According to the authors, for example, one propagation direction could be assigned 1525-1535 nm, the other could be assigned 1550-1560 nm, and the OTDR wavelength could be 1548 nm.
Optical telecommunication systems are known with wavelength division multiplexing (WDM) transmission. In these systems, a number of channels are sent, i.e. a number of mutually independent transmission signals, on the same line, usually consisting of an optical fiber, by means of optical wavelength multiplexing. The transmitted channels can be either digital or analog and are mutually distinguished because each of them is associated with a specific wavelength.
U.S. Pat. No. 5,283,686, in the name of D. R. Huber, describes, among others, optical systems including an optical amplifier and a narrow-bandwidth optical filter for removing undesired spontaneous emission. An in-fiber Bragg grating reflector reflects substantially only the input amplified signal back to an optical circulator port. The undesired emission exits from the grating reflector and is removed from the system. Cascaded grating reflectors are used in a wavelength division multiplexing (WDM) system. The article of K. Y. Chen et al., xe2x80x9cDemonstration of in service supervisory repeaterless bi-directional wavelength division multiplexing transmission systemxe2x80x9d, vol. 7, no. 9, Sep. 1, 1995, states that repeaterless long distance fiber transmission systems using erbium doped fiber amplifiers as a transmitter power amplifier and/or as a receiver preamplifier have many applications, in which cases it is infeasible or impossible to have an in line amplifier, such as island hopping and intracity links. In this letter, an in service supervisory repeaterless bi-directional six WDM channel transmission over a 200 Km single fiber link is demonstrated.
The patent application EP 0 535 590 discloses a two way repeater apparatus for directly amplifying optical signals, to be transmitted in mutually opposite directions. Said repeater receives an outward input optical signal S1 of 1.552 microns in wavelength at an input/output terminal 1, and receives an inward optical signal S1r of 1.536 microns in wavelength at an input/output terminal 20.
The patent application DE 36 32 047 A discloses a communication system used for digital narrow band and wideband signals transmitted in both directions via a single optical waveguide.
The article of M. J. Chawki et al., xe2x80x9cEvaluation of an optical boosted add/drop multiplexer OBADM including circulators and fiber grating filtersxe2x80x9d, proceeding of ECOC, vol. 1, Sep. 17, 1995, discloses two OBADM configurations wherein bi-directional EDFA and fiber grating filters are placed between the 2 circulators.
The patent application EP 0 729 248, corresponding to the patent U.S. Pat. No. 5,633,741, discloses that WDM optical fiber communications entails bi-directional transmission with at least two WDM channels in opposite transmission directions in a single fiber. Communication is by a single fiber transmission line served by bi-directional amplifiers. The amplifier includes fiber gratings that serve as filters and stabilize against oscillation due to reflections and to Rayleigh back scattering.
It is known that the wavelength bandwidth available for signals, in optical communication systems with doped-fiber amplifiers, is limited by the characteristics of the active dopant used. In the case of amplifiers doped with erbium, for example, said bandwidth is limited to the wavelength bandwidth included approximately between 1530 and 1565 nm.
The Applicant has observed that the known WDM communication systems are further limited regarding the number of channels, i.e. the independent wavelengths usable for transmission within said amplification bandwidth.
The need to reduce noise, particularly of the interferometric type, associated with the retroreflection of signals or noise toward the amplifiers, makes it imperative to maintain a good isolation between the channels at the various wavelengths propagating in the system, i.e. a good suppression of radiation at intermediate wavelengths between those of the communication channels. This isolation may be obtained, in known systems, only by maintaining a channel separation greater than a predetermined minimum value, which limits the number of channels usable in the available wavelength bandwidth. This minimum value depends on the characteristics of the components employed in the system, such as the spectral characteristics of the wavelength selective components (e.g. bandwidth, center-band attenuation, figure of merit) and wavelength stability (thermal and temporal) of the filters and of the optical signal sources.
Furthermore, to separate signals with different wavelengths, e.g. to drop some of them to receivers placed in intermediate amplification nodes or to send them, at the receiving station, to separate receivers, contiguous channels (in terms of wavelength) must be separated by more than a predetermined limit value.
Said limit value depends primarily on the characteristics of the wavelength selective components employed along the optical signal path.
By means of the present invention it is possible to transmit in an optical telecommunication system a number of independent optical channels greater than is permitted by known techniques, by employing wavelength selective components of equal characteristics.
The Applicant has found that by feeding wavelength-contiguous channels in two opposite directions in the system it is possible to use filtering, multiplexing and demultiplexing means having spectral resolution greater than the spacing between the channels.
According to a first aspect, the present invention concerns a bidirectional multichannel optical telecommunication system comprising:
means for generating optical signals suitable for generating at least three optical signals having, respectively, a first, a second and a third mutually distinct wavelengths;
a line for transmitting optical signals;
means for inputting said first and third optical signals to said transmission line, placed at a first end of said transmission line;
means for inputting said second optical signal to said transmission line, placed at a second end of said transmission line opposite from said first end; characterized by the fact that the value of said second wavelength is comprised between said first and third wavelengths.
According to another aspect, the present invention regards a multichannel optical telecommunication system for transmitting optical signals comprising:
a fiber-optic line having a first and a second end;
means for inputting, at said first end of said line, first optical signals propagating in a first direction and having a first series of mutually distinct wavelengths;
means for inputting, at said second end of said line, second optical signals propagating in a second direction, opposite to said first direction, and having a second series of wavelengths mutually distinct and distinct from the wavelengths of said first optical signals;
first means for demultiplexing said optical signals, at said second end of said line, and second means for demultiplexing optical signals at said first end of said line, said first and said second means of demultiplexing being suitable for demultiplexing signals distant from each other in wavelength by a quantity greater than or equal to a predetermined minimum value.
Characterized by the fact that the wavelengths of said first signals differ from each other by a quantity greater than or equal to said minimum value, the wavelengths of said second signals differ from each other by a quantity greater than or equal to said minimum value, while the wavelengths of said first signals differ from the wavelengths of said second signals by a quantity greater than or equal to half said minimum value.
In one of its versions, said system comprises bidirectional amplification means optically connected along said fiber-optic line. Preferably, said means comprise:
means for separating said first signals from said second signals;
means for amplifying said first signals;
means for amplifying said second signals;
means for combining said first and second signals.
Said means for separating and said means for combining include respective optical circulators.
Said means for amplifying said first and second signals may be unidirectional and may comprise respective comb filters suitable for transmitting signals at wavelengths within bands including the wavelengths of said first and said second signals, respectively, and suitable to attenuate radiation at wavelengths external to said bands. Said comb filters may comprise an optical circulator and Bragg grating reflectors cascaded at an intermediate port of said circulator.
In one version of said system, said minimum distance value is less than or equal to 1 nm.
According to a third aspect, the present invention regards a multichannel optical telecommunication system for the transmission of optical signals comprising:
a fiber-optic line;
multiplexing and demultiplexing means, for adding and dropping, in at least two different positions along said line, optical signals having mutually distinct wavelengths, said multiplexing and demultiplexing means having a spectral resolution greater than the minimum spacing between channels, characterized by the fact that channels contiguous in wavelength are fed in opposite directions along said line.
According to a fourth aspect, the present invention regards a bidirectional optical amplifier comprising:
means for inputting first optical signals propagating in a first direction and having a first series of mutually distinct wavelengths;
means for inputting second optical signals propagating in a second direction, opposite said first direction, and having a second series of wavelengths, mutually distinct and distinct from the wavelengths of said first optical signals;
means for separating said first optical signals from said second optical signals;
means for amplifying said first signals;
means for amplifying said second signals;
means for combining said first and said second signals; characterized by the fact that the wavelengths of said first and said second signals are mutually staggered.
Said means for separating and said means for combining advantageously comprise respective optical circulators.
Said means for amplifying said first and second signals may be unidirectional and may includes respective comb filters suitable for transmitting signals at wavelengths within bands comprising the respective wavelengths of said first and said second signals and suitable for attenuating radiation at wavelengths external to said bands. Said comb filters may comprise an optical circulator and Bragg grating reflectors cascaded at an intermediate port of said circulator.
According to a fifth aspect, the present invention regards a method for the bidirectional transmission of optical signals along an optical communication line comprising the steps of:
generating first optical signals at a first series of wavelengths;
generating second signals at a second series of wavelengths, different from the wavelengths of said first series;
transmitting said first signals in a first direction along the optical telecommunication line;
transmitting said second signals in a second direction along the optical telecommunication line; in which the wavelengths of said first signals are staggered with respect to the wavelengths of said second signals.
In one version, said method comprises the step of amplifying said first and said second signals along the optical communication line, which in turn preferably comprises the steps of;
separating said first from said second signals;
amplifying said first signals by means of a first optical amplifier;
amplifying said second signals by means of a second optical amplifier;
recombining said first and said second signals along said telecommunication line.